Cell surface antigens and especially mouse MHC antigens, H-2, will be studied as probes for the structure of membranes. Lateral diffusion of H-2 will be examined in single cells in order to understand how lateral diffusion rates are regulated by cells. Expression of HLA antigens in trophoblast cell membranes will be examined serologically and biochemically in order to understand the mechanisms by which these antigens are masked in development, thus protecting the fetus from rejection by its mother. Other embryo antigens, defined by teratocarcinoma cell surfaces, will be studied for their importance in early development.